Re: 18% Gray Cards - What's the idea?

I have always been a fan of the grey card. I think it comes from my early days as a TV engineer. The station I worked for was still B&W when I started so the cameras had to be set up very carefully using grey cards and all sorts of scopes and meters. Was a real pain in the @$$ but it did teach me a lot about what a scene looks like. moving into the early days of color TV was even more interesting and difficult once you added the color into the mix. I really think that has helped me in my photography

Re: 18% Gray Cards - What's the idea?

moving into the early days of color TV was even more interesting and difficult once you added the color into the mix.

Color and B&W are indeed different worlds. One meaning for our digital camera use is this:

Color work has three data color channels, red and green and blue (in our RGB system).

Grayscale has one gray channel, called Luminance (grayscale brightness).

Luminance is a computed number from this NTSC television formula: (B&W cameras recording color scenes and converting for B&W TV)

Luminosity = Red x 0.3 + Green x 0.59 + Blue x 0.11.

The idea is this is how the brightness of colors look to our eye (and B&W film), green looks bright and blue looks dim, etc. And the sum of 0.3 + 0.59 + 0.11 is 1.0. This is how B&W TV tries to reproduce the same visual brightness, called luminance. This is how the shade of red lipstick comes out looking about right in B&W pictures. Older orthochomatic film shows the lipstick as pretty much black. Panchromatic film is about showing these shades correctly. Old hat in recent years, but a very big deal in older days. Anyway, this formula is about that effect.

But if now RGB (255, 101, 80), (red is clipping), this comes out in mono as:

Data in all three RGB channels are reduced in this proportion. It is a mathematical abstraction, and is NOT the actual real data values.
This reduction hides clipping, values are no longer at 255. But the RGB sensor was clipping.

So if we look at the single gray histogram in our camera, we see value is 145, and we think all is fine in the world, just middle brightness.

But in our RGB sensor, the red channel is 255 (and probably clipping), and it needs attention to exposure. It does not affect B&W much, some loss of detail, but there is no red color to change color value (in grayscale). It is mainly a brightness thing for B&W.

So bottom line, we should always look only at all three RGB channel histograms.

Re: 18% Gray Cards - What's the idea?

Color and B&W are indeed different worlds. One meaning for our digital camera use is this:

Color work has three data color channels, red and green and blue (in our RGB system).

Grayscale has one gray channel, called Luminance (grayscale brightness).

Luminance is a computed number from this NTSC television formula: (B&W cameras recording color scenes and converting for B&W TV)

Luminosity = Red x 0.3 + Green x 0.59 + Blue x 0.11.

The idea is this is how the brightness of colors look to our eye (and B&W film), green looks bright and blue looks dim, etc. And the sum of 0.3 + 0.59 + 0.11 is 1.0. This is how B&W TV tries to reproduce the same visual brightness, called luminance. This is how the shade of red lipstick comes out looking about right in B&W pictures. Older orthochomatic film shows the lipstick as pretty much black. Panchromatic film is about showing these shades correctly. Old hat in recent years, but a very big deal in older days. Anyway, this formula is about that effect.

But if now RGB (255, 101, 80), (red is clipping), this comes out in mono as:

Data in all three RGB channels are reduced in this proportion. It is a mathematical abstraction, and is NOT the actual real data values.
This reduction hides clipping, values are no longer at 255. But the RGB sensor was clipping.

So if we look at the single gray histogram in our camera, we see value is 145, and we think all is fine in the world, just middle brightness.

But in our RGB sensor, the red channel is 255 (and probably clipping), and it needs attention to exposure. It does not affect B&W much, some loss of detail, but there is no red color to change color value (in grayscale). It is mainly a brightness thing for B&W.

So bottom line, we should always look only at all three RGB channel histograms.

I know all of this. In early TV you had to understand all of this. I later TV (Where I worked when I retired) it was so automated you really did not have to have much of a understanding of how it worked or why.

Re: 18% Gray Cards - What's the idea?

Would the issue of a clipped color channel (using the gray card) change depending on the actual colors in a scene? Or are you saying that setting your exposure with the gray card could inadvertently clip the color channel in all other shots in equal lighting conditions?

Re: 18% Gray Cards - What's the idea?

Originally Posted by Blade Canyon

Would the issue of a clipped color channel (using the gray card) change depending on the actual colors in a scene? Or are you saying that setting your exposure with the gray card could inadvertently clip the color channel in all other shots in equal lighting conditions?

No, but I'm sorry, I really don't know how to answer. I probably don't understand the question, because I don't really know which part you are asking about. But my fear is you may be trying to read too much into it. If you are able to point out any part that causes confusion, I would very much appreciate knowing which part that is, maybe it can be fixed.

The talk about clipping is really about using the camera meter to meter the subject directly, not the gray card.
For example, the talk about gray scale luminance is not about the gray card at all.

The gray card does not necessarily cause clipping. It is surely less likely to than metering direct. Because the point of metering on the gray card is to meter the actual light level in a standard situation, but specifically to be independent of the subject colors. The real point is that the gray card is to be an inexpensive substitute for an incident meter. But the meter is aimed at the gray card, not at the subject at all. We meter a value for the light that ought to then be good for many different subjects.

If metering directly on the subject, we might get clipping. Red colors (like roses outdoors) especially, because daylight white balance boosts the reds and reduces the blues. Blue things indoors (incandescent) could be the opposite. And light colors are an issue. The advantage of metering on the gray card is that it is independent of the subjects colors, so whites and yellows and greens don't read high, nor dark colors low.

Re: 18% Gray Cards - What's the idea?

Originally Posted by Jr1

Use the back of your hand

Not very accurate at all in today's workflow. Hands are way to many shades from pastie white to almost black. Also one of the other very good reasons for the grey card is when you are doing your post processing in Lightroom you can set the colors balance by clicking on the grey card. Can't do that with a hand.

Re: 18% Gray Cards - What's the idea?

Actually, references to metering on the hand specifically mean metering on the palm, which has better consistency. Not same as a gray card, and no claims of close accuracy, but it is a constant, and we could determine a normal compensation to use. About same concept as spot metering on a human face needs about +1 EV compensation.

But the real truth of the matter was that back in film days, we could get away with very many things. Even Sunny 16. B&W negatives thrived on overexposure, and regardless, all prints got required darkroom processing which helped to correct many problems.

We can post process our digital images today, otherwise they come out of camera sight unseen by human eyes. Correction of white balance and exposure, and even cropping helps greatly. Certainly raw encourages that correction, but not all bother to do that. And proper exposure in the camera is always a plus.